A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalysis contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium also comprises methyl acetate, water, methyl iodide and the catalyst. Conventional commercial processes for carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entire contents and disclosures of which are hereby incorporated by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entire content and disclosure of which is hereby incorporated by reference.
The crude acetic acid product from the reactor is processed in a purification section to remove impurities and recover acetic acid. These impurities, which may be present in trace amounts, affect the quality of acetic acid, especially as the impurities are circulated through the reaction process, which, among other things, can result in the build up of these impurities over time. Conventional purification techniques to remove these impurities include treating the acetic acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. The treatments may also be combined with the distillation of the crude acetic acid product. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
Processes for removing these impurities may also remove compounds in the reaction medium, such as the halogen promoter. Several processes have been taught for recovering the halogen promoter including treatment of vented streams and extraction.
Treatment of vented streams allows recovery of halogen promoters. For example, U.S. Publication No. 2009/0270651 discloses a methanol carbonylation system that includes an absorber tower adapted for receiving a vent gas stream and removing methyl iodide therefrom with a scrubber solvent, the absorber tower being coupled to first and second scrubber solvent sources which are capable of supplying different first and second scrubber solvents. A switching system including valves alternatively provides first or second scrubber solvents to the absorber tower and returns the used solvent and absorbed material to the carbonylation system to accommodate different operating modes.
Extraction may also recover halogen promoters from the carbonylation products. For example, U.S. Pat. No. 4,908,477 discloses separating organic iodine compounds from carbonylation products of methanol, methyl acetate and dimethyl ether and from mixtures of such carbonylation products by a process wherein the iodine compounds are removed by liquid phase extraction with a non-aromatic hydrocarbon.
The methods for recovering halogen promoters are not able to remove other impurities. In particular, the impurities that decrease the permanganate time of the acetic acid are not removed when recovering halogen promoters. Permanganate time is a quality test commonly used in the acetic acid industry to determine the amount of impurities present in an acetic acid product. These impurities include saturated and unsaturated carbonyl compounds, generally referred to as permanganate reducing compounds (PRC's). PRC's, may include, for example, compounds such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde and the like, and the aldol condensation products thereof. These compounds are removed through one or more columns and extraction units.
The art has disclosed that PRC's present in the crude acetic acid product streams generally concentrate in the overhead stream from the light ends column. Accordingly, the light ends column overhead stream has been treated with an amine compound (such as hydroxylamine), which reacts with the carbonyl compounds to form derivatives that can be separated from the remaining overhead stream by distillation, resulting in an acetic acid product with improved permanganate time.
It has been disclosed in U.S. Pat. Nos. 6,143,930 and 6,339,171, the entireties of which are incorporated herein by reference, that it is possible to significantly reduce undesirable impurities in the acetic acid product by performing a multi-stage purification on the light ends column overhead stream. These patents disclose a purification process in which the light ends overhead stream is distilled twice, in each case taking the acetaldehyde overhead and returning a methyl iodide-rich residue to the reactor. The acetaldehyde-rich distillate obtained after the two distillation steps is optionally extracted with water to remove the majority of the acetaldehyde for disposal, leaving a significantly lower acetaldehyde concentration in the raffinate that is recycled to the reactor.
In addition, it has been disclosed in US Pub. No. 2006/0247466, the entirety of which is incorporated herein by reference, that it is possible to reduce undesirable impurities in the acetic acid product by subjecting the light ends overhead stream to a single distillation to obtain an overhead stream. The overhead stream is then subjected to an extraction that selectively removes and/or reduces PRC's.
U.S. Pat. No. 7,223,886, the entirety of which is incorporated herein by reference, discloses a method for reducing the formation of alkyl iodides and C3 to C8 carboxylic acids by removing PRC's from the light phase of the condensed light ends overhead stream, including (a) distilling the light phase to yield a PRC enriched overhead stream; and (b) extracting the overhead stream with water in at least two consecutive stages and separating therefrom one or more aqueous streams containing PRC's.
The effluent of the extraction units comprises the PRC's that are removed from the acetic acid production process. The effluent may also comprise impurities that may result in a loss of the compounds in the reactor and result in effluent streams that are more expensive and difficult to treat.
While the above-described processes have been successful in reducing and/or removing PRC's from the carbonylation system, further improvements can still be made for recovering the halogen promoters.